Are you one of those guys slightly paranoid about your security? Do you think your couple, your kids or your parents could be snooping on your private WhatsApp conversations when you're not there?
If you're not worried about overcoming a confidence problem then the application Conversation Archiver for iOS devices, compatible with iPhone and iPad, is perfect for you. It allows you to archive the WhatsApp chats of your choice and only access them by means of a password or even a fingerprint if your device is compatible. But beware, with this application's security measures, it will be impossible to recover those chats if the user forgets the password to access the protected folders.
Never forget your password if you don't want your conversations to be inevitably lost.
Is it really worth paying the subscription to this application?
No, definitely not. Are you willing to pay almost 20 dollars per month to make sure nobody spies on your WhatsApp chats. I hope not, you might as well spend that money on a psychiatrist. If you're really that concerned about your security or privacy, you simply shouldn't use instant messaging for anything that you don't want to share with the NSA, the FBI, the Interpol or anyone with access to your iPhone. It's just my advice.